Spa tub



FIG. 1 is an elevated top view of a spa tub showing my new design;

FIG. 2 is an elevated right side view thereof;

FIG. 3 is an elevated left side view;

FIG. 4 is an elevated front view thereof; and,

FIG. 5 is an elevated rear view thereof.

Those portions of the figures that are presented in broken lines merely illustrate the environment of the design and are not part of the claimed design. 

The ornamental design for a spa tub, as shown and described. 